The subject matter disclosed herein relates to digital imaging systems, and particularly to increasing the dynamic range of digital X-ray detectors.
Digital X-ray imaging systems are becoming increasingly widespread for producing digital data which can be reconstructed into useful radiographic images. In current digital X-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient's tissues similar to those available through conventional photographic film techniques.
Digital X-ray imaging systems are particularly useful due to their ability to collect digital data which can be reconstructed into the images required by radiologists and diagnosing physicians, and stored digitally or archived until needed. In conventional film-based radiography techniques, actual films were prepared, exposed, developed and stored for use by the radiologist. While the films provide an excellent diagnostic tool, particularly due to their ability to capture significant anatomical detail, they are inherently difficult to transmit between locations, such as from an imaging facility or department to various physician locations. The digital data produced by direct digital X-ray systems, on the other hand, can be processed and enhanced, stored, transmitted via networks, and used to reconstruct images which can be displayed on monitors and other soft copy displays at any desired location. Similar advantages are offered by digitizing systems which convert conventional radiographic images from film to digital data.
Despite their utility in capturing, storing and transmitting image data, digital X-ray systems are still overcoming a number of challenges. For example, X-ray systems may be employed for a range of different types of examination, including radiographic and fluoroscopic imaging. Among other distinctions, these two types of imaging examinations are characterized by significantly different radiation levels used to generate the image data. Specifically, radiographic images employ substantially higher radiation levels than fluoroscopic images. In a number of applications, it may be desirable to perform both types of imaging procedures sequentially to obtain different types of data. However, current digital X-ray systems may encounter difficulties in performing both fluoroscopic and radiographic imaging with the same detector. It should be noted that such detectors are used in a number of other applications and settings. For example, in addition to projection X-ray applications, digital X-ray detectors are used for computed tomography imaging as well as for tomosynthesis imaging. Moreover, such systems are increasingly used for parcel and baggage inspection, for security systems (e.g., airport security), screening systems, industrial part inspection, and so forth.
Specifically, current digital X-ray systems employ amorphous silicon detectors with arrays of photodiodes and thin film transistors beneath an X-ray scintillator. Incident X-rays interact with the scintillator to emit light photons which are absorbed by the photodiodes, creating electron-hole pairs. The diodes, which are initially charged with several volts of reverse bias, are thereby discharged in proportion to the intensity of the X-ray illumination. The thin film transistor switches associated with the diodes are then activated sequentially, and the diodes are recharged through charge sensitive circuitry, with the charge needed for this process being measured.
However, the dynamic range (i.e., minimum and maximum exposure) of the amorphous silicon detectors is limited by the amount of charge that can be integrated in each pixel at certain exposure levels. At high exposures, saturation may occur and the signals obtained may not be representative of the number of photons or the intensity of radiation impacting individual pixel regions of the detector surface. As a result, details may be lost in the reconstructed images. As mentioned above, some of these detectors perform both radiographic and fluoroscopic imaging. Detectors used in fluoroscopy typically need the highest possible conversion factor with the lowest possible electronic noise; however, these detectors tend to have maximum exposure levels below those desirable for radiographic operation. Thus, detectors used in both radiographic and fluoroscopic operations are generally compromised as to one of these imaging operations.